1. Field of the Invention
The present invention relates to a mode-switching control device for a hybrid vehicle, the vehicle having an engine and an electric motor as on-board power sources, and being switchable between an electric operation mode (EV mode) in which the vehicle is operated with the electric motor only, and a hybrid operation mode (HEV mode) in which the vehicle is operated with the electric motor and the engine.
2. Background Information
Hybrid vehicles of this type are known in the prior art and include that disclosed, e.g., in Japanese Laid-Open Patent Application No. 2000-199442.
This hybrid vehicle has a design in which an engine provided as one of the power sources is decouplably drive-coupled to vehicle wheels via a continuously variable transmission and a clutch in that order, whereas an electric motor provided as the other power source is coupled full-time to the wheels.
With this hybrid vehicle, electric operation (EV operation) in EV mode powered by the electric motor alone is possible by stopping the engine and releasing the clutch, while hybrid operation (HEV operation) in HEV mode powered by the electric motor and the engine is possible by starting the engine and engaging the clutch.
By releasing the clutch in the aforedescribed manner during EV operation, the stopped engine (and, when a transmission is present, the transmission as well) is decoupled from the vehicle wheels, and the engine (transmission) is not co-rotated (dragged) during EV operation, avoiding energy losses to a commensurate extent, so that the energy efficiency can be increased.
In the aforedescribed hybrid vehicle, in the case of a change in the running state, such as depressing of the accelerator pedal during EV operation with the engine stopped and the clutch disengaged, the engine is restarted and the clutch is engaged, switching the mode from the EV operation mode to the HEV operation mode.
When the clutch is engaged during the EV→HEV mode-switching, the rotation speed at the input side (the engine side) thereof, being dependent on the engine rotation, is initially low. By contrast, the rotation speed at the output side (vehicle wheel side) of the clutch, being determined by the vehicle speed, is initially high.
Therefore, engagement of the clutch taking place during EV→HEV mode-switching is initiated in a state in which there is a large input/output rotation difference in the clutch, which tends to produced considerable clutch engagement shock (EV→HEV mode-switching shock).
Techniques proposed in the past for reducing clutch engagement shock include, e.g., a technique of shifting the transmission in such a way as to give a small input/output rotation difference in the clutch, as disclosed in Japanese Laid-Open Patent Application No. 2000-289471, and it would be conceivable to employ this clutch engagement shock reduction technique as a countermeasure against EV→HEV mode-switching shock in the aforedescribed hybrid vehicle.